Transient energy systems and methods for use of the same

ABSTRACT

This disclosure relates to transient energy systems for supplying power to a load substantially instantaneously on demand. Transient energy systems may include a flywheel coupled the rotor of an induction motor generator. One embodiment of the disclosure refers to systems and methods for reducing loads on a bearing in a transient energy system. In another embodiment, the disclosure refers to an induction motor generator that is optimized for high power transient power generation, yet low power motor operation. Yet another embodiment of the disclosure refers to using a flywheel as a drag pump to cool components of a transient energy system. In yet another embodiment, a slip control scheme is discussed for regulating a DC bus. In yet a further embodiment of the disclosure a method is provided for reducing unnecessary turbine starts by making turbine start a function of the rotational velocity of a flywheel.

BACKGROUND OF THE DISCLOSURE

This relates to transient energy systems for supplying power to a loadsubstantially instantaneously on demand. More particularly, this relatesto flywheel energy storage systems constructed with an inductionmotor-generator.

There are many different types of transient systems, including, forexample, batteries, ultra-capacitors, and flywheel energy storagesystems, all of which are capable of providing power substantiallyimmediately on demand. Batteries have poor reliability, are bulky (thusrequiring substantial storage space), are expensive to maintain, andcontain environmentally damaging chemicals. Ultra-capacitors areexpensive and their development has yet to achieve economies of scalewhich would permit their widespread use. An example of a flywheel energystorage system includes the relatively high mass flywheel type ofsystem. In this type of flywheel energy system, the flywheel iscontained in an airtight container under vacuum to reduce drag on theflywheel's rotation. A disadvantage of such flywheel systems is thatthey become relatively large (thereby occupying substantial floorspace), heavy and costly, in applications where longer runtimes aredesired.

Transient systems may be used in combination with a “long-term” backuppower generation system (which systems may not be capable of supplyingpower substantially immediately on demand) to form an uninterruptiblepower supply (UPS) system. A UPS system may be operative to ensure thatthere is no interruption in the supply of power to a critical load inthe event of an interruption in the supply of power from the primarypower source (e.g., a utility power failure). A critical load may be aload requiring a continuous uninterrupted supply of power such as, forexample, telecommunications systems, data centers, and other powersensitive loads. The transient energy storage system may bridge thesupply of power substantially immediately after interruption in thesupply of power from a primary power source (e.g., utility power) atleast until a “long-term” backup power generation system (e.g., acompressed air storage system) is “activated” and able to supply powerto the critical load.

Because interruptions in the supply of power from a primary power sourceare relatively rare, the need for transient systems to supply power ondemand may occur only a few times during the life of such systems.However, when backup power is needed, the demands on the transientsystems are high. The following considerations may be taken into accountfor transient system, and in particular, flywheel energy storagesystems:

(1) the operational life of the flywheel system;

(2) the power consumed during a standby mode of operation (or motoringmode of operation);

(3) the quantity of power generated during a generation mode ofoperation;

(4) the response time in which real power is generated;

(5) the heat losses generated during standby mode;

(6) the ability to cool components during standby and/or generationmodes;

(7) the wear and tear on components such as bearings;

(8) the audible noise;

(9) the range of voltage on a DC bus;

(10) the flywheel rotational speed;

(11) the stresses on the flywheel;

(12) the ability to control real power generated by the flywheel system;

(13) the control circuitry current limits; and

(14) the magnetic saturation limits.

There are several known configurations of flywheel energy storagesystems (see, for example, U.S. Pat. No. 5,731,645). These systemsmaximum energy storage and require vacuum sealed enclosures to minimizeflywheel windage losses. Some of the components within these systems canbe highly customized and require manufacturing methods that are notwidely used in industry thereby increasing cost.

Another type of system may be used to generate power, but is notconducive to transient power generation, includes conventional inductionmotor/generators that are driven by a prime mover (e.g., engine, wind,water, etc.) to generate power. Such induction motor/generators aredesigned and used for continuous duty operation. As such, the inductionmotor/generator is constructed to handle thermal and electrical limitsfor its power rating, resulting in larger sizing (and associated highercost) and limited ability to rapidly transition from a motor mode togenerator mode.

What is needed is an improved flywheel energy storage system constructedwith a low-cost induction motor/generator that is optimized fortransient power generation and methods for use of the same.

SUMMARY OF THE INVENTION

A flywheel energy storage system in accordance with the principles ofthe present invention constructed with an induction motor/generator(IMG) optimized for transient power generation is provided. The flywheelenergy storage systems according to the invention may be a verticallyaligned system that includes an induction motor/generator portion and aflywheel portion. The motor/generator portion may include statorcircuitry, which may be mounted in a non-rotational position on theframe of the motor/generator, and rotor circuitry, which may be mountedonto a rotor assembly that rotates about a vertical axis. The rotorassembly may be mounted between top and bottom bearing housingassemblies.

The flywheel energy storage system according to the invention may beused in combination with a “long-term” backup power generation systemsuch as, for example, a thermal and compressed air storage (TACAS)system for providing uninterrupted power to a critical load. In anotherexample, the flywheel energy storage system may be used in combinationwith a compressed air storage (CAS) backup power generation system.TACAS and CAS systems generate power when the compressed gas is expandedto drive a turbine, which drives a generator to produce power.

In an embodiment of the invention, a coil spring may be built into thebottom bearing housing assembly to apply an upward preload force to thebearing contained therein. This upward force counteracts a predeterminedportion of the downward force exerted by the flywheel and any othercomponent of the rotor assembly which may contribute to the downwardforce exerted on the bearing. As a result of the upward preload force,the loading on the lower bearing is reduced, thereby increasing the lifeof the bearing. If desired, the upward preload force may be such thatthe load seen by both the upper and lower bearings is approximatelyequal.

In another embodiment for reducing bearing loading, the magnetic centerof the rotor circuitry is offset in a vertical direction with respect tothe magnetic center of the stator circuitry to generate an axial force(in either an upward or downward direction depending on the direction ofthe vertical offset) when the induction motor/generator is operating ina motoring mode. This axial force may reduce the load seen by, forexample, the lower bearing, thereby increasing its operational life.

The flywheel energy storage system is optimized for transient powergeneration by reducing the physical size, standby power consumption, andinductance of the induction motor/generator, as compared to otherinduction motor/generators of comparable size. Smaller size generallycorrelates to less cost. Reduced standby power consumption saves energycost and heat losses and reduced inductance enables the flywheel systemto quickly switch from a motoring mode to a generating mode.

An advantage of the flywheel energy storage system according to theinvention is that it is used for transient operation, and therefore themotor/generator of such a system may be optimized for transient powergeneration without being limited by the constraints that affectcontinuous duty motor/generators or motor/generators operating in avacuum. That is, the IMG of the present invention may be designed tooperate at thermal and electrical limits that may cause a conventionalcontinuous duty motor/generator of the same size to fail. Such thermaland electrical levels may be achieved because the high power generationperiod is transient and the IMG is constructed to achieve the desiredperformance characteristics.

In another embodiment invention, components of the flywheel system maybe cooled using the flywheel as a single-disc Tesla pump to pull coolambient air into the flywheel system. The ambient air may travel overthe bearings, IMG rotor and stator circuitry, and other heat generatingcomponents and be expelled from exhaust ports located near the peripheryof the flywheel. Shrouding or baffling may be provided to reduce thenoise created by the air flow out of the flywheel through the exhaustports.

In another embodiment of the invention, a method is provided fordetermining the voltage applied to the induction motor/generator usingfeedback signals (e.g., DC bus voltage, motor speed, power, etc.). Thismethod enables quick adjustments to the excitation voltage without acircuit model or calculation of any current or voltage transforms bymaking instantaneous phase angle adjustments to the excitation voltagein addition to slip frequency adjustments.

In an embodiment in which the flywheel system is operating incombination with a CAS or TACAS backup energy system, systems andmethods are provided for starting the turbine only when the flywheelslows down to a predetermined rotational velocity (RPM). This is incontrast to known systems and methods that use a predetermined voltage(on the DC bus) or predetermined lapse of time after a primary powerinterruption event to trigger a turbine start event.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention, its nature andvarious advantages will be more apparent upon consideration of thefollowing detailed description, taken in conjunction with theaccompanying drawings, in which like reference characters refer to likeparts throughout, and in which:

FIG. 1 is a schematic of a backup energy system and a flywheel energystorage system in accordance with the present invention.

FIG. 2 is a schematic of another backup energy system and a flywheelenergy storage system in accordance with the present invention.

FIG. 3 is a cross-sectional view of a vertically aligned flywheel energystorage system in accordance with the present invention.

FIG. 4 is a cross-sectional view of another vertically aligned flywheelenergy storage system in accordance with the present invention.

FIG. 5 shows a three-dimensional cutaway view of a flywheel noisereduction enclosure of a flywheel system in accordance with theprinciples of the present invention.

FIG. 5A shows a more detailed three-dimensional view of portion of theflywheel noise reduction enclosure of FIG. 5 in accordance with theprinciples of the present invention.

FIGS. 5B-D show different exhaust ports that may be included in theflywheel noise reduction enclosure of FIG. 5 in accordance with theprinciples of the present invention.

FIG. 6 shows an enlarged cross-sectional view of a bearing housing forreducing a load on the bearing in accordance with the principles of thepresent invention.

FIG. 7 shows an exploded assembly view of the bearing housing of FIG. 6in accordance with the principles of the present invention.

FIG. 8 shows a partial cross-sectional view of a motor/generator portionof a flywheel energy storage system which is constructed to reduce aload applied to a bearing in accordance with the principles of thepresent invention.

FIG. 9 is a graph showing DC bus voltage versus time, power supplied tothe critical load versus time, and power generated by the flywheelenergy storage system versus time during a discharge event in accordancewith the principles of the present invention.

FIG. 10 is a graph showing an example of a failed load catch where theflywheel energy storage system fails to generate sufficient power withinthe necessary time period to power the load and replenish the DC bus.

FIG. 11 shows a graph illustrating energy, stress, and drag as afunction of flywheel speed, and further shows an energy limit, a draglimit, and a stress limit of a flywheel energy storage system designedaccording to the principles of the present invention.

FIG. 12 shows an illustrative schematic for controlling the inductionmotor/generator portion of the flywheel energy storage system inaccordance with the principles of the present invention.

FIG. 13 shows an illustrative diagram of a control scheme in accordancewith the principles of the present invention.

FIGS. 14A-C show several timing diagrams illustrating the operation of acontrol scheme in accordance with the principles of the presentinvention.

FIG. 15 shows an illustrative diagram of a prior art control scheme.

FIG. 16 is a flowchart of steps that may be taken by a control scheme inaccordance with the principles of the present invention.

FIG. 17A shows an equivalent circuit diagram for an induction motor.

FIG. 17B shows an induction motor equivalent circuit diagram simplifiedby Thevenin's Theorem.

FIG. 18 shows a timing diagram of flywheel energy and speed, turbinepower, IMG power, and load in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 shows a block diagram of a compressed air storage (CAS) backupenergy system 100, including a flywheel energy storage system inaccordance with the present invention, for providing backup power to aload. Backup energy system 100 includes utility input 110 which suppliespower to critical load 140 during normal operating conditions. Personsskilled in the art will appreciate that utility input 110 may be anysuitable type of primary power source. As illustrated in FIG. 1, backupenergy system 100 includes a flywheel energy system 102 according to theprinciples of the present invention integrated with the components of aCAS system in order to provide backup power to critical load 140. Backupenergy system 100 includes motor 120, compressor 122, gas source 126(e.g., pressure tank), valve 128, turbine 130 and electrical machine132.

During normal operating conditions, utility input 110 supplies criticalload 140 with power. Utility power 110 also provides bridging energysystem 102 with power so that it can overcome steady-state operatinglosses and store energy to be used, for example, during a power outage.Additionally, utility input 110 may power motor 120 which drivescompressor 122 such that compressed air is routed through valve 128 andstored in gas source 126. It is understood that gas source 126 may byany suitable type of compressed air reservoir, such as an undergroundsalt dome, pressure tank, or compressed air cylinders. It is furtherunderstood that if gas source 126 is a pressure tank or a compressed aircylinder, one or more pressure tanks or compressed air cylinders may beused in system 100.

Compressor 122 can be any suitable type of compressor which compacts orcompresses gas (e.g., atmospheric air) to occupy a smaller space insideof gas source 126. Valve 128 may be a conventional valve or any othersuitable device for selectively permitting or preventing the flow ofair. Moreover, rather than using a single valve 128 to direct the flowof gas from compressor 122 to pressure tank 126 and from pressure tank126 to turbine 130, two separate valves may be used. For example, onevalve may be used to route gas to a thermal storage unit (discussedbelow in connection with FIG. 2) and the other may be used to bypass thethermal storage unit.

When there is an interruption in utility power, stored energy inflywheel energy storage system 102 is used to power critical load 140during at least a portion of the interruption. As defined herein, aninterruption of power supplied by a primary power source may be aninterruption in the supply of power that may cause a load failure. Forexample, a momentary interruption (e.g., a fraction of a second) in thesupply of power may cause certain loads to fail. Depending on theseverity of the interruption, control circuitry (not shown) may causevalve 128 to open to allowed compressed gas to be routed to turbine 130.As will be explained more fully in detail below, it may not be necessaryto discharge compressed gas from the gas source 126 each time there isan interruption in the supply of primary power. For example, dependingon the length and severity of the interruption, flywheel energy storagesystem 102 may provide sufficient power to ride through theinterruption, without requiring production of power by electricalmachine 132.

When turbine 130 is driven by the compressed gas, it powers electricalmachine 132 to provide power to critical load 140. Turbine 130 may be anair turbine of any suitable topology (impulse, reaction, combustionetc.), air motor, or other compressed gas expansion engine capable ofattaining power generation speeds relatively quickly (e.g., less than asecond). Such a relatively high-speed ramp-up in power generation mayprovide a basis for enabling flywheel energy generation system 102 to bedesigned for “extreme” transient operation in accordance with theprinciples of the present invention. That is, because the turbine 130and generator 132 can begin generating power relatively quickly,flywheel system 102 may be designed to operate at current and/or thermallevels that may cause flywheel system 102 to cease functioning properlyor result in its destruction if it operates at such current and/orthermal levels beyond the time period required of the turbine 130 andelectrical machine 132 to provide power to and fully sustain criticalload 140 independent of flywheel system 102.

There may be several stages in which backup energy system 100 providespower to critical load 140 during an interruption in primary powersource 110. In a first stage, flywheel energy system 102 may be the soleenergy source from which power is derived and provided to the load. In asecond stage, both the flywheel energy system 102 and the compressed gaswhich is used to power turbine 130, which in turn powers electricalmachine 132 may provide energy from which power is derived and providedto critical load 140. In a third stage, the compressed gas which is usedto power turbine 130, which in turn powers electrical machine 132 is thesole source of energy from which power is derived and provided tocritical load 140. During the third stage, flywheel energy system 102may draw power from the electrical machine 132 to recharge (e.g., motorthe system to return the flywheel to a standby rotational speed). By atleast partially recharging flywheel energy system 102, it may be able toassist electrical machine 132 in accommodating step changes in criticalload 140 by selectively providing power to critical load 140.

FIG. 2 shows a block diagram of thermal and compressed air storage(TACAS) backup energy system 200, including a flywheel energy storagesystem in accordance with the principles of the present invention, forproviding backup power to a load. System 200 includes many of the sameelements as system 100 of FIG. 1. It is noted that, for convenience andclarity, similar components of different embodiments are similarlynumbered. For example, the gas source of FIG. 1 is numbered “126” (where1XX generally refers to elements identified in FIG. 1), while the gassource in FIG. 2 is numbered “226.” Therefore, the similarly numberedelements will not be discussed. System 200 includes a thermal storageunit 250, located between valve 228 and turbine 230. A bypass valve 229may provide a bypass path for allowing compressed gas to bypass thermalstorage unit 250. A DC bus 260, conversion circuitry 262, 264, 266, and268, and capacitor 270 may be provided. Conversion circuitry 262, 264,266, and 268 may be controlled by control circuitry (not shown). Duringnormal operation, primary power source 210 may supply power toconversion circuitry 266, which may rectify the power to provide DCpower for DC bus 260. Power on DC bus 260 may be provided to conversioncircuitry 268, which may invert the DC power to AC power for criticalload 240. Also, during the normal mode of operation, power on DC bus 260may be supplied to flywheel energy generation system 202 by way ofconversion circuitry 264. Conversion circuitry 264 may invert the DCsignal provided by DC bus 260 to a desired AC signal suitable forcontrolling the motoring function of flywheel energy generation system202.

During a backup mode of operation (e.g., when primary power source 210is interrupted), flywheel energy storage system 202 and electricalmachine 232 may provide power to DC bus 260, via conversion circuitry264 and 262, respectively. The power on the DC bus may then be providedto critical load 240 via conversion circuitry 268. Additional details ofhow the motoring and generating functions of flywheel energy generationsystem 202 operate with respect to conversion circuitry 264 and DC bus260 are discussed below.

FIG. 3 is a cross-sectional view of a vertically aligned flywheel energystorage system 300 in accordance with the principles of the presentinvention. Flywheel system 300 includes, generally, an inductionmotor/generator portion (generally shown in the top half of the FIG.)and a flywheel portion (generally shown in the bottom half of the FIG.),both of which may be contained within frame 302. The frame may be a NEMAframe of a predetermined size (e.g., NEMA 254, 256, 326, etc.). Themotor/generator portion may include stator circuitry 320 (showngenerally as laminated steel stator core 321 and stator winding 322) androtor circuitry 330 (shown generally as laminated steel rotor core 331and rotor bars 332) coupled to shaft 310. Rotor circuitry 330 as knownin the art is sometimes referred to as a squirrel-cage rotor. Statorcircuitry 320 may be permanently mounted to frame 302 and rotorcircuitry 330 may rotate about vertical axis 301 in conjunction withshaft 310. Shaft 310 may be mounted within system 300 by bearing housingassemblies 350 and 360, which include bearings 352 and 362 respectively.Bearings 352 and 362 may be grease lubricated anti-friction bearings,sleeve-type bearings, gas lubricated bearings, magnetic bearings orother suitable type of bearings.

Shaft 310, rotor circuitry 330, and flywheel 340 collectively constitutethe rotational member or assembly of flywheel system 300 and may bereferred to as the rotor or rotor assembly. The rotational membergenerally refers to the portion of system 300 that rotates aboutvertical axis 301 and may include other components not specificallymentioned herein. The rotational member may be suspended by bearingassembly 350 and supported by bearing assembly 360.

Flywheel 340 may be coupled to shaft 310 and is operative to rotateabout vertical axis 301 within chamber 342. Chamber 342 may be exposedto atmospheric pressure via exhaust ports 344 and 345. An advantage ofincluding flywheel 340 in a unitary frame construction (for containingessentially all moving elements) provides for cooling of elements (e.g.,stator and rotor circuitry and bearings) within flywheel system 300.Such cooling may be accomplished by using the spinning flywheel 340 as asingle-disc Tesla pump. When operating as a Tesla pump, air may bepulled in from the ambient environment (via air gaps in flywheel system300) and directed along the vertical axis 301 towards flywheel 340,where the air is ejected at the flywheel's outer radius, or moreparticularly exhaust ports 344 and 345. It is understood that any numberof exhaust ports may be present in chamber 342.

The rotation of shaft 310 (and flywheel 340) causes a pressure drop fromshaft 310 to the outward periphery of flywheel 340. As illustrated inFIG. 4, which shows an alternative flywheel energy storage system 400having a unitary frame construction in accordance with the principles ofthe present invention, this pressure drop produces a pumping actionwhich pulls cool, ambient air (shown by arrow lines) into system 400,from both the top and bottom portions of flywheel system 400, such thatthe air is pulled across bearings 452 and 462, and other componentswithin frame 402. The air extracts heat from the elements of system 400and carries the heat away from system 400 when expelled from one or moreexhaust ports 446 located around the periphery of flywheel 440 to theambient environment.

Note that although FIGS. 3 and 4 describe flywheel systems having aunitary frame, it is understood that various embodiments of the presentinvention may be practiced in flywheel systems having a “two-part”construction. For example, a “two-part” construction may be a systemwhere the motor/generator portion and the flywheel portion are housed inseparate housings or frames, but share a common shaft which extendsbetween both housings. The “two-part” construction may not realize thebenefit of the Tesla pump action that enables elements of themotor/generator portion to be cooled, but the Tesla pump action may beutilized to cool, for example, a bearing supporting the rotor in theflywheel portion.

A considerable amount of noise may be generated as air is ejected fromthe flywheel system due to the flywheel's pumping action. To reducenoise, the exhaust ports (e.g., ports 446) may be shaped or shrouded toreduce edge noise (of the flywheel) and flow noise produced by theairflow out of the exhaust ports. Different port shapes and sizes mayaffect the amplitude and frequency spectrum of the generated noise.

FIG. 5 shows a three-dimensional cutaway view of a flywheel noisereduction enclosure 500 of a flywheel system in accordance with theprinciples of the present invention. Enclosure 500 may be integratedwith the frame (not shown) of the flywheel system such that it formspart of a unitary frame construction as discussed above. Alternatively,enclosure 500 may be housed internal to the frame (not shown) in whichcase, the exhaust air may be vented out of ports of the frame itself.

As shown, enclosure 500 encloses flywheel 510 and includes a noisereduction structure extending around the periphery of flywheel 510. Thenoise reduction structure includes inner and outer exhaust portsarranged in multiple layers so as to create a shrouding effect thatdampens noise (e.g., flow noise and edge noise). In particular, innerexhaust ports 514 are fluidically coupled to the interior space ofenclosure 500. Outer exhaust ports 520 are fluidically coupled to anatmospheric environment (e.g., the room in which the motor/generator islocated). Outer exhaust ports 520 may be offset with respect to theposition of inner exhaust ports 514 to provide a baffle through whichthe air flow must traverse to pass from the interior to the exterior ofthe enclosure. The spatial relationship between the inner and outerexhaust ports is shown in more detail in FIG. 5A, which details thecircled region A of FIG. 5.

FIG. 5A shows inner exhaust port 514 is located between outer exhaustports 520. More particularly, FIG. 5A shows that outer exhaust ports 520are included as an outer layer 522 of enclosure 500 and inner exhaustport 514 is included as part of an inner layer 524. Inner layer 524 maybe constructed in discrete sections positioned, for example, proximal toouter exhaust ports 520. Such discrete construction may result inseveral inner exhaust ports 514. Alternatively, inner layer 524 may spancontinuously around the inside periphery of enclosure 500 (not shown),resulting in a single inner exhaust port 514 spanning around theperiphery of the flywheel. The actual construction is largely a matterof design choice though it is understood that various designs (e.g.,such as shape and size of inner and outer exhaust ports) may affect thequality of noise reduction and air flow characteristics. For example,the baffling may reduce the rate of air flow from the interior ofenclosure 500 to the ambient environment, but the baffling creates aslower, less turbulent, and therefore quieter air flow. FIGS. 5B-Dillustrate three examples of inner and outer exhaust port shapes andorientations that may be used to reduce the flow noise.

Referring back to FIG. 3, systems and methods for reducing or balancingloads of least one bearing in accordance with the principles of thepresent invention are now discussed. As discussed above, the rotationalassembly may be suspended by bearing assembly 350 (e.g., the upperbearing assembly), where the axial load due to the weight of therotational assembly is borne substantially by bearing 352, and radialloads due to unbalance forces and lateral magnetic pull between therotor and stator circuitry are shared by bearing assemblies 350 and 360.Bearing 352 is frequently referred to as the thrust bearing and isgenerally constrained to limited axial motion relative to the machinehousing. Bearing 360 is frequently referred to as the non-thrust orfloating bearing and must be free to move axially relative to themachine housing to accommodate differences in thermal expansion betweenthe rotating assembly and housing. In one embodiment, as shown in FIG.3, bearing assembly 360 may be constructed to include a spring 364 thatprovides a predetermined upward force to bearing 362. This force reducesthe load borne by bearing 352 and imparts an axial preload to bearing362 that is necessary for proper operation. In one approach, spring 364may apply an upward preload force that is approximately equal to onehalf the weight of the rotational assembly. By providing an upwardpreload force of approximately one half the weight of the rotationalassembly, the load borne by the upper and lower bearings may beequalized. This advantageously allows the upper and lower bearings to beequally sized and results in comparable life expectancies of thebearings.

Spring 364 may be a low stiffness spring (i.e., a spring with a lowforce/deflection constant). It may be desirable to provide the upwardpreload force through a relatively large deflection of a low stiffnessspring rather than a relatively small deflection of a high stiffnessspring. In particular, a large deflection of a low stiffness springminimizes the variation in preload force that results from installationtolerances and differential thermal expansion between the rotor assemblyand machine housing. For example, for a nominal rotational assemblyweight of 300 pounds, the desired upward preload force may be 150pounds. A reasonable installed deflection of the compression spring maybe 1.0 inch, which requires a spring constant of 150 lb/in. Therefore,an installation tolerance of 0.050 inches may result in a 7.5 poundvariation in the preload force (e.g., 5% of nominal). In contrast, if ahigher stiffness spring is used to apply the same upward preload force(using, for example, a spring having a 300 lb/in constant). Thus thesame installation tolerance (e.g., 0.050 inches) may result in a 15pound variation in the preload force (e.g., 10% of nominal).

FIG. 6 shows an enlarged cross-sectional view of bearing housing (e.g.,bearing housing 360 of FIG. 3) in accordance with the principles of thepresent invention. FIG. 7 shows an exploded assembly view of the bearinghousing of FIG. 6 in accordance with the principles of the presentinvention. Reference will be made to both FIGS. 6 and 7 in the followingdiscussion. As shown, housing 660 includes a bearing cartridge pocket670 in which spring 664, washer 672, and bearing 662 reside and are heldin place by bearing retainer 674. Membranes 676 (e.g., o-rings) may beused to provide a resilient mounting of bearing 662 within bearingcartridge pocket 670. When constructed, spring 664 applies an upwardforce to the washer 672, which in turn translates that upward force tothe outer race of bearing 662. As is known in the art, bearingstypically include an outer race, an inner race, and roller elements(e.g., ball bearings) fixed between the outer and inner races. Inflywheel systems according to this invention, the shaft is mounted flushagainst the inner race. Thus, the inner race rotates, while the outerrace remains stationary.

It is known that without assistance of a load reducing or load balancingsystem or method, vertical orientation of flywheel system 300 may placea majority of the weight of the rotational member (e.g., combination ofshaft 310, rotor circuitry 330, and flywheel 340) on thrust bearing 352.Thus, but for such a system or method, thrust bearing 352 may succumb towear and tear faster than floating bearing 362. It is understood thatthe principles of this invention apply if bearing 362 is used as thethrust bearing and bearing 352 is the floating bearing if the preloadspring is applied in the upward direction to the outer race of bearing352.

FIG. 8 shows a partial cross-sectional view of a motor/generator portionof a flywheel energy storage system which is constructed to reduce aload applied to a lower bearing in accordance with the principles of thepresent invention. FIG. 8 illustrates an embodiment in which adifference in magnetic centering between stator circuitry 820 and rotorcircuitry 830 may be used to create an axial force to reduce the loadapplied to a lower bearing (not shown) or equalizes a load seen by bothupper and lower bearings (not shown). As defined herein, a magneticcenter is the point of maximum magnetic coupling between the rotor andstator electrical circuits.

In conventional motor/generator arrangements, such as that shown in FIG.3, the magnetic centers of the rotor and stator circuitry may beco-aligned to maximize flux between the rotor and stator and minimizedeleterious thrust loading on bearings. In addition, such an arrangementmaximizes the rotational force that may be applied to rotate therotational assembly (only a portion of which is shown). When themagnetic centers of the rotor and stator circuitry are centered withrespect to each other on vertical axis 801, substantially no net forceis produced in a vertical direction (along vertical axis 801) when theflywheel system is operating in a motor mode or generator mode.

In FIG. 8, rotor circuitry 830 may be shifted in a vertical direction(e.g., downward) with respect to stator circuitry 820 such that magneticcenter of rotor circuitry 830 (shown as line 835) is offset with respectto the magnetic center of stator circuitry 820 (shown as line 825). Thisoffset (or vertical misalignment) of magnetic centers may induce anupward force F when the induction motor/generator is being operated as amotor. Force F has a net force component in the upward verticaldirection that serves as an axial force that “pulls” rotor circuitry 830(and by extension the rotational assembly) up, thereby reducing the loadon the lower bearing (not shown). An upward force is produced becausethe magnetic center of rotor circuitry 830 is vertically aligned belowthe magnetic center of stator circuitry 820. By contrast, if themagnetic center of rotor circuitry 830 is positioned above the magneticcenter of stator circuitry 820, a downward vertical axial force may beproduced. Thus, when the motor/generator is operating in a motor mode,there is a natural tendency for the magnetic centers to line up.Therefore, regardless of the vertical direction in the offsets ofmagnetic centers, an offset of magnetic centers results in an axialforce in a vertical direction.

The strength of the axial force may be based on a number of factors,including, for example, the magnetic field strength of rotor circuitry830 and stator circuitry 820, the offset distance, the size of themotor/generator, and any other suitable factors. The strength of theaxial force may be designed to counteract a predetermined portion of theload applied to the lower bearing (e.g., bearing 362) when the lowerbearing is being used as the thrust bearing. For example, the axialforce may be approximately equal to the load applied to a lower bearingby a rotational assembly (not shown) which may include the rotorcircuitry, shaft, and flywheel. In another example, the axial force maybe such that the load borne by both the upper and lower bearings isapproximately equal.

The motor/generator portion of the flywheel energy storage systemsaccording to the invention is constructed as an induction motorgenerator (referred to herein as an “IMG”). More particularly, the IMGis constructed to have a squirrel cage rotor.

FIG. 17A shows an equivalent circuit diagram of an inductionmotor/generator. R₁ and X₁ are the stator winding resistance and leakagereactance, respectively. R₂ and X₂ are the rotor or armature windingresistance and leakage reactance (as seen from the stator),respectively. B_(m) (also referred to herein as X_(m)) is themagnetizing reactance or the mutual reactance between the rotor andstator. Gc is a parameter representing the iron losses in the machineand is often neglected in the evaluation of the equivalent circuit. V₁is the applied stator voltage, I₁ is the total current, I_(φ) is themagnetizing or excitation current and I₂ is the rotor current as seenfrom the stator. The variable s represents the slip or relative velocitybetween the rotational speed of the rotor and the rotational speed ofthe stator magnetic field.

FIG. 17B shows the equivalent circuit diagram of FIG. 17A which as beensimplified by Thevenin's Theorem. As shown, V₁, is changed to V1 a, X1and Xm is changed to Xe1, and R1 is changed Re1. A brief description ofhow an IMG operates is now provided.

A three-phase set of sinusoidal voltages of predetermined magnitude,frequency and phase may be applied to the stator (e.g., stator 311)which causes current to flow through the stator windings. This currentis made up of 2 components, I_(φ) and I₂, which may be referred to asthe magnetizing current and torque producing current, respectively. Themagnetizing current in the stator creates a rotating stator magneticfield that passes over the rotor circuitry (e.g., bars and windings) andinduces a voltage therein. It is the relative, mechanical motion of therotor compared to the motion of the stator magnetic field that producesthe induced voltage in the rotor circuitry. This induced voltage, inturn, induces current flow through the rotor circuitry. The flowingcurrent produces a rotor magnetic field which rotates in the samedirection as the stator field.

As stated above, the voltage induced in the rotor depends on therotational speed of the rotor relative to the rotational speed of thestator field. The rotational speed of the stator field is referred to asthe synchronous speed. A term commonly used to define the rotationalspeed of the rotor relative to synchronous speed is “slip speed.” Theterm “slip” defines the relative speed in terms of a percentage orper-unit basis. For example, when the slip is 0, the rotor is rotatingat synchronous speed. When the slip is 1, the rotor is stopped. Slipspeed may be defined by equation 1.n _(slip) =n _(sync) −n _(m)  (1)wherein n_(slip) is the slip speed of the machine, n_(sync) is the speedof the magnetic fields, and n_(m) is the mechanical speed of the rotor.Slip may be defined in equation 2.s=((n _(sync) −n _(m))/n _(sync))*100  (2)

Note that the direction of the torque on the rotor is dependent on thedirection of slip. In the motoring mode, as previously discussed, therotor spins at a slower than synchronous speed resulting in a torque inthe direction of rotation that tends to restore the rotor to the fastersynchronous speed. This corresponds to a slip between 0 and 1. Whilemotoring, both torque producing current and magnetizing current arebeing consumed by the IMG. In generator mode, the rotor spins fasterthan synchronous speed resulting in a torque opposite the direction ofrotation that tends to restore the rotor to the slower synchronousspeed. This in turn corresponds to a slip between 0 and −1. In thismode, torque producing current is being supplied by the IMG butmagnetizing current is still consumed. Note that magnetizing current isconsumed in both motoring and generating modes. While magnetizingcurrent can sometimes be provided via self excitation, an externalsource, such as the capacitors of a DC bus, may be required.

The amplitude, frequency and phase of the voltages applied to the statormay determine the synchronous speed, magnitude and phase of the statorcurrent, I₁. Control circuitry may be used to control the frequency andamplitude of the magnetizing current and the amplitude of the torqueproducing component of current. By selectively varying the amplitude,frequency and phase of the applied voltages, the IMG can be selectivelytransitioned to and from motoring and generating modes with additionalcontrol over the amount of power consumed or supplied by the IMG.

The flywheel energy system according to the present invention isdesigned for transient power generation. That is, the motor/generatorportion may spend a majority of its operational life in a motoring modeand a minority in a generating mode. For example, the flywheel energysystem may be designed to discharge practically all usable energy to acritical load (and a DC bus) in a very short period of time (e.g.,ranging from a fraction of a second to a second or two). This is incontrast with continuous duty motor/generators which may generate powerfor a majority of their operational lives (e.g., a motor/generatoroperating in connection with a wind-powered turbine).

In general, the IMG of the present invention is constructed to switchfrom a low power motoring mode (e.g., standby mode) to a maximum powergenerating mode (e.g., discharge mode) within a suitably small timeframeand provide at least a minimum quantity of power to a critical load andDC bus. It is desirable for the IMG to operate in a low power motoringmode to increase efficiency and reduce heating. As will be explained ingreater detail below, power is provided by a primary power source and DCbus (e.g., DC bus 260) via conversion circuitry (e.g., conversioncircuitry 264) to the stator circuitry of the IMG during the motoringmode. The conversion circuitry may limit the quantity of power suppliedto the stator circuitry by adjusting a duty cycle (e.g., by adjustingthe duty cycle of a DC-to-AC conversion). More particularly, theconversion circuitry may reduce the voltage (or RMS voltage) provided tothe stator circuitry to reduce the amount of magnetizing current withsubsequent reduction in the amplitude of the stator field. It isunderstood that the conversion circuitry may be responsible foradjusting the frequency of the excitation current to control the slip.

It is understood that during an initial startup or a ramp up inrotational speed of the flywheel energy storage system, the conversioncircuitry may provide a higher voltage to the stator circuitry than whenthe flywheel is already at speed. Also, the conversion circuitry may beresponsible for adjusting the frequency of the excitation current tocontrol the slip.

The IMG of the present invention transitions from a low power motoringmode to a high power generating mode within a short enough timeframe toprevent the DC bus from falling below critical voltage limits andrestores the DC bus to a predetermined voltage within 1 to 3 linecycles. That is, the IMG must transition fast enough to ensure power isprovided to the critical load in the event of a disturbance in thesupply of power from the primary power source.

An advantage of the flywheel energy storage system according to theinvention is that it is used for transient operation, and therefore themotor/generator of such a system may be optimized for transient powergeneration without being limited by the constraints that affectcontinuous duty motors and generators. The response time of the IMG canbe optimized by proper design of the rotor and stator circuitryresistances and reactances. Further, the IMG of the present inventionmay be designed to operate at thermal, mechanical or electrical limitsthat may cause conventional continuous duty motors and generators of thesame size or power rating to fail. Such thermal, mechanical andelectrical levels may be achieved because the high power generationperiod is transient While it may be true that conventional continuousduty motor/generators may operate at overload conditions, the IMG of thepresent invention differs in both construction and performance ascompared to conventional continuous duty motor/generators. By way ofcomparison, these differences are now discussed.

Tables 1 and 2 compare equivalent circuit data of IMGs according to thepresent invention, labeled under column heading A, to continuous dutymotor/generators of comparable size and to IMG's of comparable powerratings. The IMG in Table 1 is designed to fit within a standard NEMA326 frame size and is compared to different continuous dutymotor/generators manufactured by Marathon Electric, A.O. Smith, andTeco-Westinghouse Motor Company. The IMG in Table 2 is then compared to2 continuous duty IMG's manufactured by Marathon Electric that havesimilar power rating but fit within a larger NEMA 404 or 405 frame size.A survey of different manufacturers and NEMA specifications indicatethat 326 frame sizes are appropriate for 50 HP (37.5 kW) and that 404frame sizes are appropriate for 125 HP (93 kW) continuous dutymotor/generators. Note that all values are presented in “per unit” basedon ratings of 100 kVA and 480 Vrms Line to Line. On a full powerdischarge, the IMG according to the present invention may provide, forexample, approximately 90 kW until the flywheel is completelydischarged, the rotor/stator steel becomes saturated, the current limitof the electronics is reached, or a thermal limit is reached.

TABLE 1 Comparison of 50 HP, 60Hz, 2 Pole Induction Motor Data MFG MARMAR MAR AOS TECO TECO A Cat # 326TSTGS6001 365USTFS4001 326TSTFS6501E3036 JM0502 DH0502 Frame 326TS 365US 326TS 326TCS 326JM 324TS 326T HP50 50 50 50 50 50 RPM 3600 3600 3600 3600 3600 3600 3600 R1 0.069 0.0670.069 0.056 0.051 0.0517 0.061 R2 0.064 0.056 0.064 0.066 0.096 0.09270.051 X1 0.512 0.493 0.512 0.257 0.394 0.3921 0.162 X2 0.43 0.439 0.4310.643 0.699 0.6956 0.1944 Xm 18.29 18.7 18.29 24.98 27.74 25.594 4.7475Toc 0.775880348 0.906567398 0.775921794 1.0298048 0.7858 0.752270.2570352 Tsc 0.038464834 0.043546715 0.038506281 0.0360664 0.0300480.03095 0.0182588

TABLE 2 Comparison of 10 HP, 60 Hz, 4 Pole Induction Motor Data MFG MARMAR Cat # P336 Y983 A Frame 256 254 256 HP 10 10 RPM 1800 1800 1800 R10.5396 0.432 0.014 R2 0.4885 0.335 0.018 X1 2.093 1.4995 0.054 X2 3.2091.741 0.040275 Xm 51.12 36.92 1.0233 Toc 0.590019031 0.6122476870.313468923 Tsc 0.056686278 0.050390752 0.026988031

Toc and Tsc are the open circuit and short circuit time constants,respectively. Ts0 is the time constant associated with changes in IMG'smagnetic fields in response to a change in voltage and assumes the slipis 0. Toc, Tsc and Ts0 are functions of the circuit parameters and arecalculated as follows:

$\begin{matrix}{T_{oc} = \frac{X_{2} + X_{m}}{2\pi\;{f \cdot R_{2}}}} & (3) \\{T_{sc} = {T_{oc} \cdot \frac{\left\lbrack {X_{1} + \frac{X_{m} \cdot X_{2}}{X_{m} + X_{2}}} \right\rbrack}{X_{1} + X_{m}}}} & (4) \\{T_{s\; 0} = {\frac{\left\lbrack {X_{e\; 1} + X_{2}} \right\rbrack}{R_{e\; 1}} = \frac{\left\lbrack \frac{X_{m} \cdot \left( {R_{1}^{2} + {X_{1} \cdot X_{m}} + X_{1}^{2}} \right)}{R_{1}^{2} + \left( {X_{1} + X_{m}} \right)^{2}} \right\rbrack + X_{2}}{\left\lbrack \frac{X_{m}^{2} \cdot R_{1}}{R_{1}^{2} + \left( {X_{1} + X_{m}} \right)^{2}} \right\rbrack}}} & (5)\end{matrix}$where f refers to electrical frequency of the applied voltage.

The transient response characteristics of the machine are functions ofits reactances and resistances. These parameters may be altered bychanging the design and construction of the IMG. Magnetizing reactancecan be reduced by decreasing the number of turns, reducing machinevolume, increasing the air gap size. Rotor leakage reactance can bereduced by modifying the bar/slot shape and size, reducing the machinevolume and increasing the number of bars/slots, increasing the air gapsize. Stator leakage reactance can be reduced by reducing the number ofturns, reducing the machine volume and keeping the slot size small. Itis further recognized that these reactances can be reduced on atransient basis by selective implementation of shorted conductors orshields in close proximity to the windings. The rotor and statorresistances can be decreased by decreasing the winding length (fewerturns, shorter machine length, smaller slot pitch etc.), increasing theconductor area (either larger conductors or smaller conductors inparallel) or changing the conductor material.

A comparison of the data in both tables reveals several notabledifferences. For example, the magnetizing reactance of the invention isapproximately ¼ that of similar sized IMG's and approximately ½ that ofIMG's with similar power rating. Also, the time constants of theinvention are substantially lower than those of continuous duty motorsof comparable size and power rating. Most importantly, Ts0, whichprovides an indication for the amount of time it takes to build up themagnetic fields in the IMG is approximately half the value for IMG's ofcomparable size and power rating. Lastly, it is clear from the maximumtorque values that the overload capability of the invention iscomparable to the larger motors.

It clear from equations 3 and 5 that increasing resistances R₁ and R₂ isalso a means of reducing the time constants. However, increasing thesevalues too much can result in poor machine efficiency and excessheating. Therefore, adjustments in R₁ and R₂ must be considered in lightof the expected operating environment and chosen thermal managementsystem. It is also clear from equations 3 through 5 that, lowering themagnetizing and leakage reactances enables the IMG to more quicklyswitch from a motoring mode to a generating mode. The reactances (X₁,X₂, X_(m)) can be scaled by frequency to obtain inductances (L₁, L₂,L_(m)) that provide an indicator of the energy stored within themagnetic fields of the IMG during operation. Inductance represents theability of an electrical circuit to store energy in the form of amagnetic field. The presence of inductive energy storage in a circuitmay cause a slowed or lagging buildup of current in response to anapplied voltage. Increases in inductance generally lead to slowerresponse (e.g., switch from a motor mode to a generator mode) and,therefore, it is preferable to minimize inductances in designs wherequick response is needed to meet performance goals.

In addition to slower response times, increased inductance may requirethat more energy be supplied from an external source in order to buildup magnetic fields needed to operate the IMG. The IMG of the presentinvention may be constructed to operate in connection with a DC busthat, in turn, provides power to a critical load via conversioncircuitry during a discharge event. In the event that the primary powersource is disconnected, the DC bus voltage will begin to fall as currentflows through the conversion circuitry to the critical load. Asdiscussed above, the IMG's of the present invention are designed tooperate at reduced power levels during a motoring mode to limit powerlosses and operating temperatures. Operating at such low power levelsmay result in a relatively low magnetization current which may be toolow to enable the IMG to generate full power output immediately upontransition from motor mode to generation mode. Therefore, energy mustalso be drawn from the DC bus to build the magnetic fields within theIMG upon entry into discharge. Thus, the DC bus capacitance must besized appropriately to provide power to the critical load and providethe energy needed to build up the magnetic fields within the IMG whileavoiding excessive voltage fluctuations during entry into discharge. Thepresent invention is designed to limit capacitance requirements for theDC bus by minimizing the energy stored within magnetic fields andminimizing response time. This, in turn helps to reduce system cost andpackaging requirements due to reduction in the physical number and sizeof capacitors on the DC bus.

Note that there may be critical voltage limits associated with the DCbus. These limits may include, for example, the voltage below which thecritical load may cease to function properly and the voltage below whichthe IMG fails to produce sufficient power to supply the load andrecharge the DC bus. Load voltage sensitivity is highly variabledepending on the equipment that makes up the load. Typical values are inthe range of 10%-40% below nominal. In the present embodiment, theminimum required voltage needed to generate full power on the currentembodiment of this design is 360 Vrms, line to line. Below this voltage,the internal impedance (combined internal reactances and resistances) ofthe machine may limit the current flow within the IMG to the point whereit can no longer perform at full capacity.

In addition to meeting a range of DC bus voltage requirements, the IMGis designed to meet performance requirements for a range of flywheelspeeds. The flywheel speed range can be limited by several factorsincluding energy storage requirements, material property limits, dragloss requirements, and packaging constraints. The operational speedrange of the flywheel may be optimized based on these factors. Once theflywheel speed range is set, the IMG may be designed to operate at fullpower over that range and preferably a reasonable tolerance beyond thatrange The DC bus voltage can vary widely depending on load requirementsand utility voltage available at the installation site. In oneembodiment, the IMG may be designed to operate with a DC bus havingvoltages ranging between 360 and 540 VDC. It is understood that the IMGcan be designed to operate in connection with other voltage ranges, andneed not be limited by the specific embodiment discussed herein.

Design limitations for the IMG when taking the flywheel speed range intoaccount include (1) mechanical stresses on the rotor at maximum speed(2) lowest specified DC bus voltage at maximum speed and (3) highest DCbus voltage at minimum speed. The first limit may be used to determinethe maximum diameter of the IMG in order to maintain acceptable stresslevels in the rotor. The second limit may be used to tune the circuitparameters of the IMG such that the breakdown torque (or sometimesreferred to as pushover torque) of the machine is some suitably smallmargin above that required to meet output power requirements. Thebreakdown torque is the point when an IMG is overloaded beyond the IMG'storque capability which causes the IMG to stall or abruptly slow down.The third limit the may be used to tailor the size of the IMG such thatthe IMG reaches magnetic saturation and/or as the peak current limits ofthe converter electronics are reached. In this embodiment, the flywheelmay fail to produce adequate power to recharge the DC bus and criticalload at a DC bus voltage less than 360 Vdc and a flywheel speed of 8000rpm and may saturate or reach electronics current limits at a DC busvoltage of 480 Vdc and 3600 RPM. It is understood that the IMG can bedesigned to operate in connection with other voltage and flywheel speedranges, and need not be limited by the specific embodiment discussedherein.

FIG. 9 is a graph showing DC bus voltage versus time, power supplied tothe critical load versus time, and power generated by the IMG versustime during a discharge event in accordance with the principles of thepresent invention. As illustrated in FIG. 9, the DC bus initiallysupplies all of the power to the critical load from T0 to T1, which isthe time period from the initial interruption in the supply of powerfrom a primary power source to the critical load (T0) to a trigger pointto start discharge of the IMG (T1). Between time periods T0 and T1,current supplied from the DC bus to the critical load causes the voltageon the DC bus to fall, as shown. The trigger point (shown here as T1)may occur when certain parameters are met. For example, initiation ofdischarge may occur when the DC bus voltage drops to or below apredetermined voltage level, when the rate of change in voltage of theDC bus exceeds a predetermined rate or when the flywheel speed dropsbelow a given RPM threshold.

Starting at time period T1, the IMG begins generating power which issupplied to the critical load. The generation of power may slow the rateof change in the decrease of DC bus voltage and begin charging the DCbus. Time period T2 represents a point in the discharge period where theIMG begins to charge the DC bus. Thus, from time period T2 onward (atleast until all practical usable flywheel energy is consumed, or acurrent or thermal limit is reached), the IMG may generate enough powerto supply the critical load and recharge the DC bus voltage. FIG. 9illustrates a discharge event where the IMG successfully “catches” theload. In contrast, FIG. 10 illustrates an example of a failed load catchwhere the IMG fails to generate sufficient power within the necessarytime period to power the load and replenish the DC bus. As a result, andas shown, the DC bus voltage may continue to decline and eventuallydecline to a point where the internal impedance of the IMG limitscurrent flow below that needed to meet magnetizing current requirementsand to support the load and DC bus. The flywheel system according to thepresent invention is constructed and operative to prevent such failures.

In one aspect of the present invention, the flywheel of the flywheelsystem may be designed to balance the following factors: energy, stress,drag, weight, speed, where it is desirable to minimize stress,aerodynamic drag, and weight while maximizing stored energy. FIG. 11shows a graph illustrating energy, stress, and drag a function of speed.FIG. 11 further shows an energy limit, a drag limit, and a stress limitof a flywheel designed according to the principles of the presentinvention. Stress is proportional to the square of tip speed (i.e.,linear velocity of the flywheel periphery), and aerodynamic drag is acomplicated function of the cube of tip speed and other geometricfactors. Balancing the negative effects of high tip speed on stress anddrag with the positive effects on stored energy per unit weight requiresan optimization process in which many tradeoffs are necessary,especially considering that lower weight requires higher tip speed toachieve desired energy production. As shown in FIG. 11, the result of anoptimization process according to the invention yields an acceptablerotor speed range that falls inside of the stress and drag limitations,but that exceeds the energy storage requirements of the flywheel system.

The flywheel system may be used both as an energy source and an energysink (e.g., to prevent a turbine overspeed condition). Generally, airturbines, such as turbine 130 of FIG. 1, may not be capable ofsubstantially immediately responding to changing load conditions. Suchchanges in load demand may be referred to herein as sudden step changesin load demand. This may be due to delays associated with valve actionbut also the presence of gas (e.g., air) in the system havinginappropriate thermodynamic states (e.g., pressure and temperature) forthe new load requirements. Therefore, an IMG according to the inventionmay be used as a short-term energy source in the event of a step load ora short-term energy sink in the event of a step unload. An example ofhow the flywheel energy system may be used as both an energy source andan energy sink is now discussed in connection with FIG. 18, which showsa timing diagram of flywheel energy and speed, turbine power, IMG power,and load in accordance with the principles of the present invention.

Starting at time t⁻¹, when line power is providing power to the load,the energy level and the flywheel speed may be at a maximum or standbylevel and speed, respectively, and the turbine and IMG are not providingpower because such power is not required. At time to, an interruption inline power occurs, at which point, the IMG provides power to the load.As shown, as the IMG continues to provide power, the flywheel energy andspeed decrease. Up until time t₁ (time between t₀ and t₁), the IMG isthe only source of backup power. At time t₁, the turbine is activated(e.g., the compressed gas system is activated to supply power to theload), and both the IMG and turbine are supplying power to the load. Attime t₂, the turbine is the primary source of backup power provided tothe load. That is, the IMG is no longer required to supply thesubstantial portion of the backup power to the load. Note that theflywheel speed is now at an intermediate speed. As will be explained,from time t₂ onward, the IMG operates to compensate for sudden changesin load demand.

At time t₂, even though the IMG may not be providing power, its flywheelcontinues to operate at an intermediate speed, thereby maintaining atleast an intermediate energy level. At time t₃, a step unload eventoccurs. During a step unload event, the load demand may suddenly drop,thereby resulting in a situation where the turbine is temporarilysupplying too much power (because the response time of the compressedenergy system may not be able to instantaneously track changes in loaddemand). This excess power may be absorbed by or “sunk” into the IMG.When absorbing the power, the IMG may use it to speed up the rotationalspeed of the flywheel, as shown, resulting in a higher intermediateflywheel speed. The flywheel energy may also increase when absorbingpower from the turbine.

At time t₄, a step load event occurs. During this event, the IMG maysupplement power provided by the turbine until the compressed gas systemadjusts to the new load demand. During load events, the flywheel speedmay decrease to a lower intermediate speed, as shown. Note that althoughthe energy level and flywheel speed of the IMG changes to variousintermediate speeds, the IMG may remain operative to supply and absorbpower for subsequent changes in load demand.

At time t₅, another step unload event occurs. Then, at time t₆, linepower is restored, at which point the power provided by the turbine isno longer needed. The compressed gas system may shut down and the excesspower may be absorbed by the IMG to bring, for example, the flywheelspeed up to or near a standby speed.

When used as an energy source during a step load, the IMG temporarilyprovides power to the load from the energy stored in the flywheel. Whenused as an energy sink during a step unload, the IMG temporarily absorbsexcess power generated by the turbine-powered generator as the turbinetransitions to a lower power level. Use of the IMG as a sink and sourceadvantageously eliminates a need to use another energy sink, such as aload resistor, to compensate for step unload events and an additionalenergy source, such as a battery, to compensate for a step load event.Sinking excess power to the IMG may prevent a turbine overspeedcondition which could potentially damage the turbine.

It is understood that although the use of the IMG as a source and a sinkis discussed as operating in connection with a compressed gas system,the IMG may be used with other systems capable of providing backuppower.

FIG. 12 shows an illustrative block diagram of control circuitry 1210,IMG 1220, converter circuitry 1230, DC bus 1240, utility power 1250 inaccordance with the principles of the present invention. As shown, IMG1220 is electrically coupled to converter 1230, which is electricallycoupled to DC bus 1240. DC bus 1240 may be electrically coupled toutility power 1250 via converter circuitry 1255 and to a load (notshown). Communications paths 1260 may be provided to allow control anddata signals to be transmitted to, from, and between various components(e.g., control circuitry 1210, IMG 1220, converter circuitry 1230, andDC bus 1240). The data signals may include rotational speed, torque,temperature, and other data relating to IMG 1220 and the voltage of DCbus 1240. The control signals may be converter control signal (e.g., aPWM signal). Converter 1230 may provide data signals (e.g., voltage,current, and frequency) to control circuitry 1210.

Converter 1230 may be any suitable combination of circuitry capable ofAC-to-DC conversion. For example, converter 1230 may include IGBTs,transistors, silicon controlled rectifiers, and/or diodes. Converter1230 may be responsive to a converter control signal provided by controlcircuitry 1210 to provide an IMG control signal to IMG 1220.

Control circuitry 1210 may be operative to provide a control signal(e.g., a PWM control signal) to converter 1230 based on the datareceived from IMG 1220, converter 1230, the DC bus, or other datasource. In general, control circuitry 1210 may directly control theoperation of converter 1220 to control flow of real and reactive powerto and from IMG 1220. The flow of real and reactive power to and fromIMG 1220 may be determined by frequency and phase control 1212 andamplitude control 1214. Frequency and phase control 1212 may dictate howmuch real and reactive power is provided to and/or extracted from IMG1220. For example, in a motor mode, both real and reactive power may beprovided to IMG 1220. In a generation mode, reactive power may beprovided to IMG 1220, whereas real power is extracted from IMG 1220.Amplitude control 1214 may dictate the quantity of real and reactivepower provided to and/or extracted from IMG 1220. For example, duringthe motor mode, amplitude control 1214 may provide a signal that resultsin relatively low power consumption by IMG 1220. During the generationmode, amplitude control 1214 may provide a signal that results in a highpower output by IMG 1220. The signal provided by amplitude control 1214may be based on a number of factors, including but not limited to, adesired magnetization current and load demand.

An advantage of the present invention is that control circuitry 1210 maybe operative to control the slip to regulate voltage on DC Bus 1240.Control circuitry 1210 may exercise such control by substantiallyinstantaneously adjusting the frequency of the IMG control signal (e.g.,excitation voltage) provided to IMG 1220 (e.g., the stator), where thetransient response in any frequency change is substantially increased byusing a proportional phase jump to instantaneously shift the phase ofthe IMG control signal. Such frequency control can be performed withouta circuit model or calculation of any current or voltage transforms.

FIG. 13 shows an illustrative control scheme 1300 that may beimplemented in accordance with an embodiment of the present inventionfor controlling slip of an IMG. Control scheme 1300 may be implemented,for example, in control circuitry 1200 of FIG. 12. Control scheme 1300may receive several inputs, including a target bus voltage signal(labeled as “V_(DC) Set Point”), a real-time or continuously measuredvalue of the DC Bus (labeled as “V_(DC) Feedback”), mechanical (rotor)frequency (labeled as A), and amplitude control signal (labeled as B)(provided, for example, by amplitude control 1214). The mechanicalfrequency of the rotor may be obtained, for example, by using atachometer.

V_(DC) Set Point may be provided by control circuitry and in oneembodiment, the control circuitry may vary the set point depending onthe operational mode of the IMG. For example, in a standby motoringmode, V_(DC) Set Point may be set to a lower voltage than when the IMGis operating in a power generating mode. In another embodiment, theV_(DC) Set Point may be permanently fixed to a predetermined voltagelevel.

Operation of the control scheme 1300 is now discussed. At differentiator1302 (or sometimes referred to as a comparator), a difference between“V_(DC) Set Point” and “V_(DC) Feedback” is obtained and provided to aproportional integral (PI) controller 1310, which generates a slipdemand, which is provided as the PI controller output. Slip demandrefers to the slip that needs to exist between the machine rotorfrequency and the electrical frequency to ensure the desired DC busvoltage is maintained. PI controllers are known in art and need not bediscussed in great detail. As shown, PI controller 1310 is a simplifiedversion of a controller including gain constants K1 and K2 (where theconstants may have the same or different values), integrator 1312, andadder 1314.

When “V_(DC) Feedback” is less than “V_(DC) Set Point,” a negative slipdemand may be generated by PI controller 1310. A negative slip demandmay be generated when the IMG is in the generating mode or needs totransition from a motoring mode to a generating mode. For example,“V_(DC) Feedback” may drop below “V_(DC) Set Point” when primary powerfails and the load draws power from the DC Bus. When power is drawn fromthe DC Bus, the voltage may decrease. This decrease in voltage causes PIcontroller 1310 to generate a negative slip, which may causes the IMG tosupply real power to the load while simultaneously charging the DC Bus.

When “V_(DC) Feedback” is greater than or equal to “V_(DC) Set Point,” apositive slip demand may be generated by PI controller 1310. A positiveslip may be generated when the IMG is in a motoring mode or needs totransition from a generating mode to a motor mode.

The slip demand is added to mechanical frequency/speed at adder 1320 toprovide an electrical stator frequency. For example, by rearrangingequation 1 above, it is understood that adding slip speed to mechanicalspeed yields a stator electrical frequency. This stator electricalfrequency is integrated by integrator 1322 to produce a phase anglesweep at the stator frequency. This integrated term may be provided toadder 1330.

The slip demand, provided by PI controller 1310 may be scaled by a gain,K3, to produce a phase angle jump proportional to the slip demand (or ascaled slip demand). The phase angle jump (or scaled slip demand), whenadded to integrated frequency at adder 1330, substantially improves thetransient response time of control scheme 1300. This is because thephase angle jump can cause the phase angle of the integrated frequencyto “jump” into the new phase demanded of the slip prior to the end of aswitching cycle. Thus inclusion of gain K3 adds a parallel controlsignal to control scheme 1300 that enables this embodiment of thepresent invention to more quickly adjust the phase angle of theintegrated frequency than prior art control schemes.

The integrated frequency having the adjusted phase angle may beconverted into a sinusoidal signal at sine function generator 1340. Thissinusoidal signal (which may be the signal provided by frequency andphase control 1212 of FIG. 12) may be used to control the flow of realand reactive power into and/or out of an IMG. Optionally, an amplitudecontrol signal (which may be the signal provided by amplitude control1214 of FIG. 12) may be mixed (e.g., multiplied) at mixer 1350 toproduce a converter control signal (e.g., a PWM duty signal).

FIGS. 14A-C show several timing diagrams illustrating the operation ofan IMG control scheme in accordance with the principles of the presentinvention. FIG. 14A shows the phase angle of stator frequency and a sinewave representative of the phase angle. Prior to time, t, the slope ofthe phase angle is relatively constant. Assume, for purposes ofdiscussion, that the IMG is operating in a generation mode. As such, thestator frequency is less than the rotor rotational frequency. At time t,a change in “V_(DC) Feedback” occurs (e.g., a step unload event occurs)which causes IMG control scheme to change the phase angle (as evidencedby increase in slope of the phase angle), signifying an increase instator frequency. In this example, assume “V_(DC) Feedback” increased involtage, meaning that the load demand on the IMG decreased. As a resultof the increase in “V_(DC) Feedback,” the slip demand increases, whichcauses the stator frequency to increase (relative to rotor mechanicalfrequency), thereby reducing the power generated by the IMG. Theresponsiveness of the control scheme is shown by the rapid change in theslope of the phase angle.

FIG. 14B compares a sine waveform generated by an IMG control schemeaccording to the invention to a sine waveform generated by a prior artIMG control scheme when the stator frequency increases. FIG. 15 shows aprior art control scheme from which the sine waveform is generated. Asshown, prior to time t, both the prior art sine waveform and the sinewaveform according to the invention are the same. At time t, where afrequency change occurs, it is seen that the sine waveform according tothe invention changes to a desired phase angle faster than the prior artwaveform. FIG. 14C compares a sine waveform generated by an IMG controlscheme according to the invention to a sine waveform generated by aprior art IMG control scheme when the stator frequency decreases. Attime t, it is also seen that the sine waveform according to theinvention changes to a desired phase angle faster than the prior artwaveform.

Since the response time of IMG control scheme is substantially fasterthan the prior art, several advantages are realized, including use ofsmaller capacitors, enhanced load catching ability, and lower costs.

FIG. 16 is a flowchart showing steps that may be taken to control an IMGin accordance with the principles of the present invention. At step1610, a slip demand may be provided. As discussed above, the slip demandmay be generated based on a net difference between a measured DC busvoltage level (e.g., “V_(DC) Feedback”) and a desired DC bus voltagelevel (e.g., “V_(DC) Set Point”). At step 1620, a stator frequency isgenerated based on the slip demand and a received mechanical frequency.The stator frequency may be the frequency required to restore the DC busback to the desired DC bus voltage level. At step 1630, the statorfrequency may be integrated to provide a phase angle sweep at the givenfrequency. At step 1640, the slip demand may be scaled to provide ascaled slip demand. The scaled slip demand provides an additionalcontrol parameter (which is generated in parallel to the integratedstator frequency) that enhances the response time of the IMG controlscheme according to the invention, as shown in step 1650. That is, thescaled slip demand may be used to adjust the phase angle of the statorfrequency so that the desired phase angle is rapidly obtained.

FIGS. 13, 14, and 16 may be used in connection with any IMG, not justthe transient IMG according to embodiments of the present invention. Forexample, the control scheme may be used in connection with continuousIMGs.

In one aspect of the present invention, a method is provided forreducing the number of unnecessary turbine starts. The flywheel in theflywheel system according to the invention may be sized and rotated atsufficient speed such that it stores enough energy for the system todeliver full power to a load for a slightly longer time than the timeneeded to bring the turbine (e.g., turbine 130 of FIG. 1) to speed. Theextra stored energy (i.e., the stored energy in excess of that needed todeliver full power for the time needed to bring the turbine to fullpower) may enable the flywheel system to support certain disturbances inthe supply of primary power without requiring turbine startup. Thishelps preserve the life of a variety of moving parts associated with theturbine (e.g., valves, regulators, turbine bearings, etc.).

Conventionally, turbine operation is initiated after a fixed time delay.If the power disturbance is ongoing after the fixed time delay, theturbine is initiated to supply power to the critical load. However, inmany instances, this conventional method may result in unnecessaryturbine start events. For example, consider a situation where loadconsumption is less than the rated power output of the flywheel systemand the primary power source is disturbed for a period of time which mybe ridden through by the flywheel system, but the turbine is activatednonetheless because the fixed time delay is exceeded. In accordance withthe present invention, the turbine (e.g., turbine 130 of FIG. 1) may bestarted when the flywheel of flywheel system slows down to apredetermined rotational velocity (RPM). This approach ensures that theturbine is started only when absolutely necessary. That is, the turbineis activated when the predetermined rotational velocity is reached suchthat it is able to get up to speed in time to provide power to thecritical load before the energy stored in the flywheel energy system isdepleted. Thus, this approach activates the turbine based on an energylevel of the flywheel system, as opposed to a fixed time.

The above described embodiments of the present invention are presentedfor purposes of illustration and not of limitation, and the presentinvention is limited only by the claims which follow.

1. An uninterruptible backup power supply system that provides power toa critical load, the system comprising: a flywheel energy storage systemcomprising a flywheel which rotates about an axis; a compressed gassystem comprising a source of compressed gas, a turbine, and agenerator; and control circuitry operative to: monitor the rotationalspeed of the flywheel; and when the monitored rotational speed slowsdown to a predetermined rotational speed, release gas from the source ofcompressed gas to drive the turbine, wherein the flywheel system isoperative to supply backup power to the critical load in the event of adisruption in the supply of power from a primary power source to thecritical load.
 2. The system of claim 1, wherein the compressed gassystem is operative to supply backup power to the critical load when theturbine is driving the generator.
 3. The system of claim 1, wherein theflywheel speed decreases when the flywheel energy storage systemprovides backup power to the critical load.
 4. The system of claim 1,the control circuitry operative to: allow the flywheel energy storagesystem to temporarily provide power to the critical load withouttriggering a turbine start event.
 5. The system of claim 1, wherein,while the compressed gas system provides backup power to the criticalload, and in the event of an increased step change in load demand, theflywheel system is operative to provide backup power to the criticalload at least until the compressed gas system compensates for the stepchange in load demand, and wherein the flywheel rotational speeddecreases to a lower intermediate rotational speed as a result of thestep change in load demand.
 6. The system of claim 1, wherein, while thecompressed gas system provides backup power to the critical load, and inthe event of an decreased step change in load demand, the flywheelsystem is operative to absorb power from the compressed gas system, andwherein the flywheel rotational speed increases to a higher intermediaterotational speed as a result of the step change in load demand.
 7. Thesystem of claim 1, wherein the flywheel is operative to maintain aneffective energy level to compensate for increases and decreases in stepchanges in load demand.
 8. A method for determining turbine startup inan uninterruptible backup power supply system comprising a flywheelenergy storage system and a compressed gas system, the methodcomprising: monitoring the flywheel speed of the flywheel energy storagesystem; and when the monitored flywheel speed slows down to apredetermined speed, initiating turbine startup of a turbine of thecompressed gas system, wherein monitoring the flywheel speed comprisesmonitoring the flywheel speed while the flywheel energy storage systemis providing power from a primary power source to the critical load. 9.The method of claim 8, wherein the flywheel speed decreases when theflywheel energy storage system provides backup power to the criticalload.
 10. The method of claim 8, wherein initiating turbine startupcomprises: releasing gas from a source of compressed gas; providing thereleased gas to the turbine; and driving the turbine with the releasedgas.
 11. The method of claim 8, further comprising providing backuppower from the compressed gas system to the critical load prior to fullydepleting energy storage of the flywheel energy storage system.
 12. Themethod of claim 8, further comprising: allowing the flywheel energystorage system to temporarily provide power to the critical load withouttriggering a turbine start event.
 13. A method for compensating for stepchanges in a load demand in an uninterruptible backup power supplysystem comprising a flywheel energy storage system and a compressed gassystem, the method comprising: using the compressed gas system toprovide backup power to a load; monitoring step changes in the loaddemand, the step changes comprising sudden step changes; using theflywheel energy system to assist the compressed gas system incompensating for step changes in the load by supplying power to the loadin the event of a sudden increase in load demand and absorbing powerproduced by the compressed gas system in the event of a sudden decreasein load demand.
 14. The method of claim 13, further comprising: rotatinga flywheel of the flywheel energy system at an intermediate speed whilethe compressed gas system provides backup power to the load.
 15. Themethod of claim 14, wherein the rotational speed decreases to a lowerintermediate speed when power is temporarily provided by the flywheelenergy system to supplement power provide by the compressed gas system.16. The method of claim 14, wherein the rotational speed increases to ahigher intermediate speed when excess power provided by the compressedgas system is absorbed by the flywheel energy system.
 17. The method ofclaim 13, further comprising: maintaining a rotational speed of aflywheel of the flywheel energy generation system at a speed effectivefor compensating for increases and decreases in load demand.